Edited that mistake out. It might also be a matter of external perception. When one sings "Sunday Bloody Sunday" or "We Are The World", people treat it as fake fuzzy drivel that tastes like diabetes at best. "Darned Beatniks (or insert some other inaccurate label here), they don't understand how the world works!".

Religious people, on the other hand (especially those belonging to very popular religions or religions you are supposed to believe in), seem to be exempt from this perception: no matter how outlandish and naïve they can get, people will admire them for believing in the face of overwhelming evidence, and for not letting failure and injustice and persecution bring them down.

Martyrs are a particularly extreme version of this, one Abrahamics seem to love.

Perhaps a part of the source of the humour of "The Book Of Mormon" is that Mormonism is "mainstream" (that is, Christian) enough to be recognized as something that it's noble to believe in, yet unorthodox enough that some of its tenets will seem absurd to most other denominations. If it were a religion that we, the audience, were not familiar with at all, and that had no connection to the faiths, the affiliations, that we were born unto... No matter how otherwise popular, it wouldn't be nearly as funny. In fact, it would be perceived as a cruel mockery.